1. Field of the Invention
This invention relates to method and apparatus for the analysis, by chromatography, of anions in a sample solution.
2. Description of the Prior Art
The term "ion chromatography" is used to described a highspeed chromatography directed mainly to inorganic ions which was disclosed by H. Small et al in 1975. It has already been reduced to practice and has been finding extensive use in applications to various forms of microanalysis, such as analysis of ecological specimen. The inventors of the present invention, pursued their studies independently of the ion chromatography method discussed above, and have developed their own ion chromatography system (hereinafter referred to as "IC" system for short), which far surpassed the aforementioned ion chromatography system disclosed by Small et al. A patent application covering the "IC" system has already been filed under the title "Method and Apparatus for the Analysis of Anions in Sample Solution".
FIG. 1 is an explanatory diagram illustrating the construction of the aforementioned IC system, cited as a conventional example. The IC system, as illustrated in the diagram, is provided with an eluant solution reservoir 1, for storing an eluant solution which is an aqueous solution containing Na.sub.2 CO.sub.3 /NaHCO.sub.3 in a concentration of the order of several mM/liter; a pump 2, for transferring, under pressure, the eluant solution, such as, to sample injection means 3; a sample injection means 3, for admitting (or automatically collecting) a prescribed amount of the sample solution delivered, such as, with the aid of a microsyringe and, at the same time, conveying this sample solution with the eluant solution from pump 2; a separation column 4, packed with anion-exchange resin; a decationizer means 5, formed of a first compartment for receiving the eluant solution from separation column 4, a second compartment for receiving a scavenger solution, and a wall of a perfluorocarbon sulfonic acid type cation-exchange composition, such as NAFION (trademark for a product of DuPont) serving as a common partition between the first and second compartments; detector means 6, for receiving the eluant from the first compartment in decationizer means 5 and, at the same time, measuring the conductivity of the eluant solution; a recorder 7, for displaying a chromatogram in accordance with an output signal from the detector 6; a scavenger solution reservoir 8, for storing a scavenger solution formed of a prescribed solvent, such as, for example, dodecylbenzene sulfonic acid; a pump 9, for transferring, under pressure, the scavenger solution from scavenger solution reservoir 8, to the second compartment in decationizer means 5; a reservoir 10 for storing a liquid already measured and flowing out of detector 6; and a reservoir 11, for storing the scavenger solution flowing out of the second compartment of decationizer means 5. The separation column 4, decationizer means 5, and detector 6 are, more often than not, kept in a constant temperature bath 12, which is maintained at a prescribed temperature.
In the conventional ion chromatography (IC) system, above descibed, when the eluant solution Na.sub.2 CO.sub.3 /NaHCO.sub.3 in eluant solution reservoir 1, is transferred via pump 2, sample injection means 3, and separation column 4, to decationizer means 5, it is converted to H.sub.2 CO.sub.3 at decationizer means 5, in consequence of cation exchange of Na.sup.+ and H.sup.+. The aqueous solution of this H.sub.2 CO.sub.3 retains a state of equilibrium as indicated by the following formula (1) ##STR1## and has a certain degree of conductivity. For example, the conductivity of an eluant solution of 4 mM Na.sub.2 CO.sub.3 /4 nM NaHCO.sub.3, when measured after having passed through decationizer means 5, is found to be about 20 to 30 .mu.s/cm.
When a sample solution composed preponderantly of water is introduced from sample injection means 3, the ions in the sample solution are retained in separation column 4, for a prescribed length of time and the water is not retained in separation column 4, but is passed through separation column 4. This water is not affected in any way either in decationizer means 5, but is passed therethrough. Of all the components of the sample solution, water is the first to reach detector 6.
On the other hand, the eluant solution exiting from decationizer means 5, contains carbonic acid, as described above, and has a conductivity of about 20 to 30 .mu.s/cm. When water reaches detector 6 while the eluant solution is still in detector 6, and recorder 7 is drawing a base line of a chromatogram, the conductivity is lowered by the water. Consequently, a negative peak begins to appear in the chromatogram. This peak is ascribable to the water. This phenomenon, thus, is called a "water dip".
When the ion chromatograph system is used for experiment by injecting 100 .mu.l of a sample solution having water as a main component and containing 50 ppb of F.sup.-, 100 ppb of Cl.sup.-, 150 ppb of NO.sub.2.sup.-, 300 ppb of PO.sub.4.sup.3-, 100 ppb of Br.sup.-, 300 ppb of NO.sub.3.sup.-, and 400 ppb of SO.sub.4.sup.2- (hereinafter referred to as "experiment solution") through sample injection means 3, a chomatogram, such as shown, in FIG. 2, is obtained on recorder 7.
From FIG. 2, it is noted that while F.sup.- and Cl.sup.- barely produce output signals of the order of only 0.002 to 0.003 .mu.s/cm per ppb, water produces a peak output signal of as high as 0.8 .mu.s/cm, indicating that water has a significant effect upon the chromatogram. The peak of water shows the so-called tailing phenomenon. Due to this phenomenon, coupled with the fact that the peaks of F.sup.- and Cl.sup.-, which have brief retention periods, appear immediately after the peak of the water, there arises a significant problem, namely, that the highly sensitive measurements of F.sup.- and Cl.sup.- become infeasible. To avoid the various above discussed problems, there has been suggested a method which eliminates the "water dip" by adding to the sample solution, a prescribed amount of Na.sub.2 CO.sub.3 /NaHCO.sub.3, in advance thereby substantially equalizing the Na.sub.2 CO.sub.3 /NaHCO.sub.3 concentration in the sample solution and that in the eluant solution.
This method, however, has a disadvatange in that, the Na.sub.2 CO.sub.3 / NaHCO.sub.3 reagent to be used must be tested for its purity, in advance of the addition to the sample solution , and the sample solution itself must be used in large amounts. The so-called concentration column method involves injecting a large amount of the sample solution, allowing all the anions in the sample solution to accumulate in a concentration column, and consequently enabling the measurement of the anions to be effected with sensitivity 10 to 100 times the ordinary sensitivity. When the Na.sub.2 CO.sub.3 /NaHCO.sub.3 is added to the sample solution until the concentration thereof equals that in the eluant solution, however, this method cannot be used, because the anions subjected to measurement are no longer retained in the concentration column. Furthermore, since practically all of the sample solutions given to be analyzed by ion chromatography have water as their main component, there is great demand for a solution to the above discussed and other problems of "water dip".
There exists a problem apart from the above discussed problem of water dip. When a sample, such as an organic specimen, which contains a trace amount of NO.sub.2.sup.- in conjunction with a large amount of Cl.sup.-, is analyzed for trace anion, accurate measurement of the trace anion becomes difficult because of the peak of trace anion (such as NO.sub.2.sup.-) is either affected abnormally or prevented from appearing at all by the interference offered by the peak of the large amount of anion (such as Cl.sup.-).